The present invention relates to an improvement of a pulse width modulator circuit (hereinafter, referred to as a PWM circuit). PWM circuits have been widely used in a pulse width modulation type amplifier, a switching regulator and so on.
FIG. 1 is a block diagram of a prior art PWM circuit having an output pulse duty ratio limiting capability. A PWM main part 5 is constructed of an input terminal 1, an amplifier circuit 7, a sawtooth wave oscillator circuit 8, a comparing section 6 composed of comparators 9 and 10 and an AND circuit 11, an output amplifier circuit 12, and an output terminal 2. The potential difference between a power supply line 3 and a reference potential line 4 is divided by serially connected resistors 13 and 14 to apply a reference voltage to a non-inverting terminal 17. The operation of the circuit shown in FIG. 1 will be explained with reference to FIG. 1 and a timing chart in FIG. 2. Here, a duty ratio D of the pulses is defined to be a ratio of the period T.sub.H when the pulse is at a high level to the cycle period T.sub.o of the sawtooth wave, as represented by Equation (1): EQU D=(T.sub.H /T.sub.o) (1)
An input analog signal applied to the input terminal 1 is amplified by the amplifier circuit 7 and then applied as a voltage V.sub.15 to a non-inverting input terminal 15 of the comparator 9 to be compared with the output V.sub.16 of the sawtooth wave oscillator circuit 8 applied to an inverting input terminal 16. As a result, PWM pulses V.sub.18 appear at a comparator output line 18. On the other hand, a voltage V.sub.17 obtained by dividing a potential difference between the power supply line 3 and the reference potential source 4 by the resistors 13 and 14 is applied to a non-inverting input terminal 17 of the comparator 10, while the output of the sawtooth wave oscillator circuit 8 is applied to the inverting input terminal 16. Consequently, pulses V.sub.19 having a fixed duty ratio Do appear at an output terminal 19 of the comparator 10. This fixed ratio D.sub.o is determined by the voltage V.sub.17 applied to the non-inverting input terminal 17. That is, the fixed ratio D.sub.o is the ratio of the time period when the voltage V.sub.17 is higher than the voltage V.sub.16 of the sawtooth wave to the cycle period T.sub.o of the sawtooth wave. The PWM pulses V.sub.18 and the pulses V.sub.19 are applied respectively to the input of the AND circuit 11 to produce pulses V.sub.20. The pulses V.sub. 20 have pulse widths modulated by the input signal at the input terminal and have duty ratios equal to or less than the fixed duty ratio D.sub.o. After the pulses V.sub.20 are amplified by the output amplifier circuit 12, a pulse width modulation output is derived from the output terminal.
The comparing section 6 composed of the comparators 9 and 10 and the AND circuit 11 as shown in FIG. 1, may be constructed as shown in FIG. 3 with a constant current source 21, transistors 22, 23, 24, 28 and 30 and resistors 25, 26, 27 and 29 as shown in FIG. 3. This is equivalent to a 3-input comparator having non-inverting input terminals 15 and 17, an inverting input terminal 16 and an output terminal 20. Among inputs applied to the two terminals 15 and 17, a lower input potential is compared with the potential applied to the inverting input terminal 16. One detailed example of the sawtooth wave oscillator circuit 8 is illustrated in FIG. 4. The circuit shown in FIG. 4 consists of a comparator 40, transistors 32 and 36, resistors 31, 34, 35, 37, 38 and 39 and a capacitor 33. The capacitor 33 is charged through the resistor 31 to gradually increase the potential at the terminal 16 when the potential at the terminal 16 is lower than the potential at the connection point of the resistors 34, 35 and 37. On the other hand, when the potential at the terminal 16 is higher than the potential at the connection point, the capacitor 33 is discharged through the transistor 32 which is made conductive by the output of the comparator 40. In response to the discharge, the potential at the terminal 16 drops rapidly. Thereafter, the charging into the capacitor 33 resumes. Accordingly, the sawtooth wave shown in FIG. 2 is obtained as a voltage at the terminal 16.
The PWM circuit having a pulse duty ratio limiting function can protect a switching element or the like, which is provided in the output amplifier circuit or a circuit connected thereafter, from breakdown when a large input signal having an amplitude higher than the sawtooth wave is applied to the terminal 15. More specifically, if the PWM circuit does not provide the pulse duty ratio limiting function, a large input signal will make the switching element conductive for more than a cycle period of the sawtooth wave, and may finally drive it into a thermal breakdown. By limiting the output pulse duty ratio, since the continuously conductive state of the switching element can be prevented, the switching element is protected from thermal breakdown.
However, this pulse duty ratio limiting function cannot protect the switching element from breakdown due to large transients during power-on. During the time immediately after the power is turned on, voltages at many circuit points are not voltages at steady state, and a large current is apt to flow for a long time through the switching element. This is another cause of the switching element's breakdown, but could not be prevented by the prior art.